wait.c

  1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * Generic waiting primitives.
  4 *
  5 * (C) 2004 Nadia Yvette Chambers, Oracle
  6 */
  7#include "sched.h"
  8
  9void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *name, struct lock_class_key *key)
 10{
 11	spin_lock_init(&wq_head->lock);
 12	lockdep_set_class_and_name(&wq_head->lock, key, name);
 13	INIT_LIST_HEAD(&wq_head->head);
 14}
 15
 16EXPORT_SYMBOL(__init_waitqueue_head);
 17
 18void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 19{
 20	unsigned long flags;
 21
 22	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
 23	spin_lock_irqsave(&wq_head->lock, flags);
 24	__add_wait_queue(wq_head, wq_entry);
 25	spin_unlock_irqrestore(&wq_head->lock, flags);
 26}
 27EXPORT_SYMBOL(add_wait_queue);
 28
 29void add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 30{
 31	unsigned long flags;
 32
 33	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
 34	spin_lock_irqsave(&wq_head->lock, flags);
 35	__add_wait_queue_entry_tail(wq_head, wq_entry);
 36	spin_unlock_irqrestore(&wq_head->lock, flags);
 37}
 38EXPORT_SYMBOL(add_wait_queue_exclusive);
 39
 40void add_wait_queue_priority(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 41{
 42	unsigned long flags;
 43
 44	wq_entry->flags |= WQ_FLAG_PRIORITY;
 45	spin_lock_irqsave(&wq_head->lock, flags);
 46	__add_wait_queue(wq_head, wq_entry);
 47	spin_unlock_irqrestore(&wq_head->lock, flags);
 48}
 49EXPORT_SYMBOL_GPL(add_wait_queue_priority);
 50
 51int add_wait_queue_priority_exclusive(struct wait_queue_head *wq_head,
 52				      struct wait_queue_entry *wq_entry)
 53{
 54	struct list_head *head = &wq_head->head;
 55
 56	wq_entry->flags |= WQ_FLAG_EXCLUSIVE | WQ_FLAG_PRIORITY;
 57
 58	guard(spinlock_irqsave)(&wq_head->lock);
 59
 60	if (!list_empty(head) &&
 61	    (list_first_entry(head, typeof(*wq_entry), entry)->flags & WQ_FLAG_PRIORITY))
 62		return -EBUSY;
 63
 64	list_add(&wq_entry->entry, head);
 65	return 0;
 66}
 67EXPORT_SYMBOL_GPL(add_wait_queue_priority_exclusive);
 68
 69void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 70{
 71	unsigned long flags;
 72
 73	spin_lock_irqsave(&wq_head->lock, flags);
 74	__remove_wait_queue(wq_head, wq_entry);
 75	spin_unlock_irqrestore(&wq_head->lock, flags);
 76}
 77EXPORT_SYMBOL(remove_wait_queue);
 78
 79/*
 80 * The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just
 81 * wake everything up. If it's an exclusive wakeup (nr_exclusive == small +ve
 82 * number) then we wake that number of exclusive tasks, and potentially all
 83 * the non-exclusive tasks. Normally, exclusive tasks will be at the end of
 84 * the list and any non-exclusive tasks will be woken first. A priority task
 85 * may be at the head of the list, and can consume the event without any other
 86 * tasks being woken if it's also an exclusive task.
 87 *
 88 * There are circumstances in which we can try to wake a task which has already
 89 * started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
 90 * zero in this (rare) case, and we handle it by continuing to scan the queue.
 91 */
 92static int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
 93			int nr_exclusive, int wake_flags, void *key)
 94{
 95	wait_queue_entry_t *curr, *next;
 96
 97	lockdep_assert_held(&wq_head->lock);
 98
 99	curr = list_first_entry(&wq_head->head, wait_queue_entry_t, entry);
100
101	if (&curr->entry == &wq_head->head)
102		return nr_exclusive;
103
104	list_for_each_entry_safe_from(curr, next, &wq_head->head, entry) {
105		unsigned flags = curr->flags;
106		int ret;
107
108		ret = curr->func(curr, mode, wake_flags, key);
109		if (ret < 0)
110			break;
111		if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
112			break;
113	}
114
115	return nr_exclusive;
116}
117
118static int __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int mode,
119			int nr_exclusive, int wake_flags, void *key)
120{
121	unsigned long flags;
122	int remaining;
123
124	spin_lock_irqsave(&wq_head->lock, flags);
125	remaining = __wake_up_common(wq_head, mode, nr_exclusive, wake_flags,
126			key);
127	spin_unlock_irqrestore(&wq_head->lock, flags);
128
129	return nr_exclusive - remaining;
130}
131
132/**
133 * __wake_up - wake up threads blocked on a waitqueue.
134 * @wq_head: the waitqueue
135 * @mode: which threads
136 * @nr_exclusive: how many wake-one or wake-many threads to wake up
137 * @key: is directly passed to the wakeup function
138 *
139 * If this function wakes up a task, it executes a full memory barrier
140 * before accessing the task state.  Returns the number of exclusive
141 * tasks that were awaken.
142 */
143int __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
144	      int nr_exclusive, void *key)
145{
146	return __wake_up_common_lock(wq_head, mode, nr_exclusive, 0, key);
147}
148EXPORT_SYMBOL(__wake_up);
149
150void __wake_up_on_current_cpu(struct wait_queue_head *wq_head, unsigned int mode, void *key)
151{
152	__wake_up_common_lock(wq_head, mode, 1, WF_CURRENT_CPU, key);
153}
154
155/*
156 * Same as __wake_up but called with the spinlock in wait_queue_head_t held.
157 */
158void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr)
159{
160	__wake_up_common(wq_head, mode, nr, 0, NULL);
161}
162EXPORT_SYMBOL_GPL(__wake_up_locked);
163
164void __wake_up_locked_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
165{
166	__wake_up_common(wq_head, mode, 1, 0, key);
167}
168EXPORT_SYMBOL_GPL(__wake_up_locked_key);
169
170/**
171 * __wake_up_sync_key - wake up threads blocked on a waitqueue.
172 * @wq_head: the waitqueue
173 * @mode: which threads
174 * @key: opaque value to be passed to wakeup targets
175 *
176 * The sync wakeup differs that the waker knows that it will schedule
177 * away soon, so while the target thread will be woken up, it will not
178 * be migrated to another CPU - ie. the two threads are 'synchronized'
179 * with each other. This can prevent needless bouncing between CPUs.
180 *
181 * On UP it can prevent extra preemption.
182 *
183 * If this function wakes up a task, it executes a full memory barrier before
184 * accessing the task state.
185 */
186void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode,
187			void *key)
188{
189	if (unlikely(!wq_head))
190		return;
191
192	__wake_up_common_lock(wq_head, mode, 1, WF_SYNC, key);
193}
194EXPORT_SYMBOL_GPL(__wake_up_sync_key);
195
196/**
197 * __wake_up_locked_sync_key - wake up a thread blocked on a locked waitqueue.
198 * @wq_head: the waitqueue
199 * @mode: which threads
200 * @key: opaque value to be passed to wakeup targets
201 *
202 * The sync wakeup differs in that the waker knows that it will schedule
203 * away soon, so while the target thread will be woken up, it will not
204 * be migrated to another CPU - ie. the two threads are 'synchronized'
205 * with each other. This can prevent needless bouncing between CPUs.
206 *
207 * On UP it can prevent extra preemption.
208 *
209 * If this function wakes up a task, it executes a full memory barrier before
210 * accessing the task state.
211 */
212void __wake_up_locked_sync_key(struct wait_queue_head *wq_head,
213			       unsigned int mode, void *key)
214{
215        __wake_up_common(wq_head, mode, 1, WF_SYNC, key);
216}
217EXPORT_SYMBOL_GPL(__wake_up_locked_sync_key);
218
219/*
220 * __wake_up_sync - see __wake_up_sync_key()
221 */
222void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode)
223{
224	__wake_up_sync_key(wq_head, mode, NULL);
225}
226EXPORT_SYMBOL_GPL(__wake_up_sync);	/* For internal use only */
227
228void __wake_up_pollfree(struct wait_queue_head *wq_head)
229{
230	__wake_up(wq_head, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE));
231	/* POLLFREE must have cleared the queue. */
232	WARN_ON_ONCE(waitqueue_active(wq_head));
233}
234
235/*
236 * Note: we use "set_current_state()" _after_ the wait-queue add,
237 * because we need a memory barrier there on SMP, so that any
238 * wake-function that tests for the wait-queue being active
239 * will be guaranteed to see waitqueue addition _or_ subsequent
240 * tests in this thread will see the wakeup having taken place.
241 *
242 * The spin_unlock() itself is semi-permeable and only protects
243 * one way (it only protects stuff inside the critical region and
244 * stops them from bleeding out - it would still allow subsequent
245 * loads to move into the critical region).
246 */
247void
248prepare_to_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
249{
250	unsigned long flags;
251
252	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
253	spin_lock_irqsave(&wq_head->lock, flags);
254	if (list_empty(&wq_entry->entry))
255		__add_wait_queue(wq_head, wq_entry);
256	set_current_state(state);
257	spin_unlock_irqrestore(&wq_head->lock, flags);
258}
259EXPORT_SYMBOL(prepare_to_wait);
260
261/* Returns true if we are the first waiter in the queue, false otherwise. */
262bool
263prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
264{
265	unsigned long flags;
266	bool was_empty = false;
267
268	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
269	spin_lock_irqsave(&wq_head->lock, flags);
270	if (list_empty(&wq_entry->entry)) {
271		was_empty = list_empty(&wq_head->head);
272		__add_wait_queue_entry_tail(wq_head, wq_entry);
273	}
274	set_current_state(state);
275	spin_unlock_irqrestore(&wq_head->lock, flags);
276	return was_empty;
277}
278EXPORT_SYMBOL(prepare_to_wait_exclusive);
279
280void init_wait_entry(struct wait_queue_entry *wq_entry, int flags)
281{
282	wq_entry->flags = flags;
283	wq_entry->private = current;
284	wq_entry->func = autoremove_wake_function;
285	INIT_LIST_HEAD(&wq_entry->entry);
286}
287EXPORT_SYMBOL(init_wait_entry);
288
289long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
290{
291	unsigned long flags;
292	long ret = 0;
293
294	spin_lock_irqsave(&wq_head->lock, flags);
295	if (signal_pending_state(state, current)) {
296		/*
297		 * Exclusive waiter must not fail if it was selected by wakeup,
298		 * it should "consume" the condition we were waiting for.
299		 *
300		 * The caller will recheck the condition and return success if
301		 * we were already woken up, we can not miss the event because
302		 * wakeup locks/unlocks the same wq_head->lock.
303		 *
304		 * But we need to ensure that set-condition + wakeup after that
305		 * can't see us, it should wake up another exclusive waiter if
306		 * we fail.
307		 */
308		list_del_init(&wq_entry->entry);
309		ret = -ERESTARTSYS;
310	} else {
311		if (list_empty(&wq_entry->entry)) {
312			if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
313				__add_wait_queue_entry_tail(wq_head, wq_entry);
314			else
315				__add_wait_queue(wq_head, wq_entry);
316		}
317		set_current_state(state);
318	}
319	spin_unlock_irqrestore(&wq_head->lock, flags);
320
321	return ret;
322}
323EXPORT_SYMBOL(prepare_to_wait_event);
324
325/*
326 * Note! These two wait functions are entered with the
327 * wait-queue lock held (and interrupts off in the _irq
328 * case), so there is no race with testing the wakeup
329 * condition in the caller before they add the wait
330 * entry to the wake queue.
331 */
332int do_wait_intr(wait_queue_head_t *wq, wait_queue_entry_t *wait)
333{
334	if (likely(list_empty(&wait->entry)))
335		__add_wait_queue_entry_tail(wq, wait);
336
337	set_current_state(TASK_INTERRUPTIBLE);
338	if (signal_pending(current))
339		return -ERESTARTSYS;
340
341	spin_unlock(&wq->lock);
342	schedule();
343	spin_lock(&wq->lock);
344
345	return 0;
346}
347EXPORT_SYMBOL(do_wait_intr);
348
349int do_wait_intr_irq(wait_queue_head_t *wq, wait_queue_entry_t *wait)
350{
351	if (likely(list_empty(&wait->entry)))
352		__add_wait_queue_entry_tail(wq, wait);
353
354	set_current_state(TASK_INTERRUPTIBLE);
355	if (signal_pending(current))
356		return -ERESTARTSYS;
357
358	spin_unlock_irq(&wq->lock);
359	schedule();
360	spin_lock_irq(&wq->lock);
361
362	return 0;
363}
364EXPORT_SYMBOL(do_wait_intr_irq);
365
366/**
367 * finish_wait - clean up after waiting in a queue
368 * @wq_head: waitqueue waited on
369 * @wq_entry: wait descriptor
370 *
371 * Sets current thread back to running state and removes
372 * the wait descriptor from the given waitqueue if still
373 * queued.
374 */
375void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
376{
377	unsigned long flags;
378
379	__set_current_state(TASK_RUNNING);
380	/*
381	 * We can check for list emptiness outside the lock
382	 * IFF:
383	 *  - we use the "careful" check that verifies both
384	 *    the next and prev pointers, so that there cannot
385	 *    be any half-pending updates in progress on other
386	 *    CPU's that we haven't seen yet (and that might
387	 *    still change the stack area.
388	 * and
389	 *  - all other users take the lock (ie we can only
390	 *    have _one_ other CPU that looks at or modifies
391	 *    the list).
392	 */
393	if (!list_empty_careful(&wq_entry->entry)) {
394		spin_lock_irqsave(&wq_head->lock, flags);
395		list_del_init(&wq_entry->entry);
396		spin_unlock_irqrestore(&wq_head->lock, flags);
397	}
398}
399EXPORT_SYMBOL(finish_wait);
400
401int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
402{
403	int ret = default_wake_function(wq_entry, mode, sync, key);
404
405	if (ret)
406		list_del_init_careful(&wq_entry->entry);
407
408	return ret;
409}
410EXPORT_SYMBOL(autoremove_wake_function);
411
412/*
413 * DEFINE_WAIT_FUNC(wait, woken_wake_func);
414 *
415 * add_wait_queue(&wq_head, &wait);
416 * for (;;) {
417 *     if (condition)
418 *         break;
419 *
420 *     // in wait_woken()			// in woken_wake_function()
421 *
422 *     p->state = mode;				wq_entry->flags |= WQ_FLAG_WOKEN;
423 *     smp_mb(); // A				try_to_wake_up():
424 *     if (!(wq_entry->flags & WQ_FLAG_WOKEN))	   <full barrier>
425 *         schedule()				   if (p->state & mode)
426 *     p->state = TASK_RUNNING;			      p->state = TASK_RUNNING;
427 *     wq_entry->flags &= ~WQ_FLAG_WOKEN;	~~~~~~~~~~~~~~~~~~
428 *     smp_mb(); // B				condition = true;
429 * }						smp_mb(); // C
430 * remove_wait_queue(&wq_head, &wait);		wq_entry->flags |= WQ_FLAG_WOKEN;
431 */
432long wait_woken(struct wait_queue_entry *wq_entry, unsigned mode, long timeout)
433{
434	/*
435	 * The below executes an smp_mb(), which matches with the full barrier
436	 * executed by the try_to_wake_up() in woken_wake_function() such that
437	 * either we see the store to wq_entry->flags in woken_wake_function()
438	 * or woken_wake_function() sees our store to current->state.
439	 */
440	set_current_state(mode); /* A */
441	if (!(wq_entry->flags & WQ_FLAG_WOKEN) && !kthread_should_stop_or_park())
442		timeout = schedule_timeout(timeout);
443	__set_current_state(TASK_RUNNING);
444
445	/*
446	 * The below executes an smp_mb(), which matches with the smp_mb() (C)
447	 * in woken_wake_function() such that either we see the wait condition
448	 * being true or the store to wq_entry->flags in woken_wake_function()
449	 * follows ours in the coherence order.
450	 */
451	smp_store_mb(wq_entry->flags, wq_entry->flags & ~WQ_FLAG_WOKEN); /* B */
452
453	return timeout;
454}
455EXPORT_SYMBOL(wait_woken);
456
457int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
458{
459	/* Pairs with the smp_store_mb() in wait_woken(). */
460	smp_mb(); /* C */
461	wq_entry->flags |= WQ_FLAG_WOKEN;
462
463	return default_wake_function(wq_entry, mode, sync, key);
464}
465EXPORT_SYMBOL(woken_wake_function);